Q1. a) Consider a single-phase transformer whose equivalent circuit is given below with primary and secondary turns of 1000 and 200 respectively. 16.ΧΩ 25. Υ Ω 4.YQ 5.Χ Ω 20 kV 50Hz 20.Χ ΚΩ 10 ΚΩ Fig 1: Transformer equivalent circuit (i) Draw the simplified equivalent circuit by referring series components on the primary side to the secondary giving all component values. (ii) The transformer is connected to a resistive load of 60. Y, calculate the current through the load. (iii) Given the load in part (ii) calculate the total loss in the transformer and hence the efficiency. Discuss whether the transformer is optimised for efficiency at this load.

For an ideal transformer, the efficiency is (a) 0 (b) 100 (c) 50

Consider an ideal transformer with N1=3000andN2=1000 turns. Let winding 1 be connected to a source whose voltage is e1(t)=100(1| t |)volts for 1t1ande1(t)=0 for | t |1 second. A2- farad capacitor is connected across winding 2. Sketch e1(t),e2(t),i1(t),andi2(t) versus time t.

For a short-circuit test on a 2-winding transformer, with one winding shorted, can you apply the rated voltage on the other winding? (a) Yes (b) No

The ratings of a three-phase three-winding transformer are Primary(1): Y connected 66kV,15MVA Secondary (2): Y connected, 13.2kV,10MVA Tertiary (3): A connected, 2.3kV,5MVA Neglecting winding resistances and exciting current, the per-unit leakage reactances are X12=0.08 on a 15-MVA,66-kV base X13=0.10 on a 15-MVA,66-kV base X23=0.09 on a 10-MVA,13.2-kV base (a) Determine the per-unit reactances X1,X2,X3 of the equivalent circuit on a 15-MVA,66-kV base at the primary terminals. (b) Purely resistive loads of 7.5 MW at 13.2 kV and 5 MW at 2.3kV are connected to the secondary and tertiary sides of the transformer, respectively. Draw the per- unit impedance diagram, showing the per-unit impedances on a 15-MVA,66-kV base at the primary terminals.

A 23/230-kV step-up transformer feeds a three-phase transmission line, which in turn supplies a 150-MVA,0.8 lagging power factor load through a step-down 230/23-kV transformer. The impedance of the line and transformers at 230kVis18+j60. Determine the tap setting for each transformer to maintain the voltage at the load at 23 kV.

The symbol shown is a(n) a. iron core transformer. b. auto transformer. c. current transformer. d. air core transformer.

Consider the three single-phase two-winding transformers shown in Figure 3.37. The high-voltage windings are connected in Y. (a) For the low-voltage side, connect the windings in , place the polarity marks, and label the terminals a, b, and c in accordance with the American standard. (b) Relabel the terminals a, b, and c such that VAN is 90 out of phase with Va for positive sequence.

With the American Standard notation, in either a Y-or-Y transformer, positive- sequen quantities on the high-voltage side shall lead their corresponding quantities on the low-voltage side by 30. (a) True (b) False

The ideal transformer windings are eliminated from the per-unit equivalent circuit of a transformer. (a) True (b) False